Efficiency is an important figure of merit for a power amplifier (PA). Better efficiency increases battery life of a handset or reduces power consumption of radio base stations. With new communication standards such as Long Term Evolution (LTE) emerging, more and more frequency bands are allocated within the same regions or sites. Hence, interest in telecom equipment operating at multiple frequency bands is growing.
Moreover, digital modulation schemes for high data rates exhibit large amplitude dynamic range, with peak to average power ratios (PAPR's) in the order of 6 to 10 dB, meaning that the utilized amplifier should be operated at a reduced power; not only to be linear to faithfully transmit the modulation, but also in order to have enough power capability to handle those “rarely occurring” amplitude peaks within the amplitude envelope of the RF (Radio Frequency) signal.
Amplifiers typically exhibit their highest efficiency when operated in a saturated condition (i.e. with no available power headroom). If the PA “size” is dynamically adapted to the signal being amplified, this condition can be maintained over the amplitude dynamic range, thus improving efficiency for high PAPR signals. There exist a number of efficiency enhancement techniques to achieve such size adaption of the PA, such as dynamic drain modulation, where the DC supply voltage is varied, often referred to as envelope tracking (ET), and dynamic load modulation, where the terminating load impedance to the PA is varied with the signal envelope. Varying the load impedance for a fixed supply voltage changes the maximum available RF current swing, and thus the available power headroom, and hence maintains high efficiency over larger signal dynamic range.
However, both the drain modulator and the load modulator required in above concepts consume DC power, without contributing to any RF output power, thus impacting the total system efficiency.
Another, more common efficiency enhancement technique is therefore the Doherty principle, originally proposed by W. H. Doherty in 1936, where two separate amplifiers are combined without isolation, thereby affecting each other's load impedance. At higher power levels a peaking (or secondary) amplifier starts to work in parallel to a main (or primary) amplifier, thereby reducing the load impedance seen by the main amplifier, simultaneously contributing to output power in the higher power regimes.
The Doherty amplifier has been adjusted to support multiple frequency bands. For example, X. Li, W. Chen, Z. Zhang, Z. Feng, X. Tang, and K. Mouthaan, “A Concurrent Dual-Band Doherty Power Amplifier,” Asia-Pacific Microw. Conf., Yokohama, Japan, December 2010, pp. 654-657 presents a novel Doherty power amplifier (PA) that realizes concurrent dualband operation. A T-network is used to implement a dual-band impedance transformer and a phase shifter simultaneously. To prove the concept a PA is designed operating at 900 MHz and 2000 MHz simultaneously. However, the presented solution necessitates dual-band modifications in at least four places of the output combining network in order to operate in the desired frequencies. This necessitates significant work to support any new or modified frequency.